Simple Interest Principal Calculator – Free Online Tool
Calculate the original principal from your target end balance using simple interest. Fast, accurate, and mobile-friendly.
Looking to find how much money you need to start with to reach a certain amount under simple interest? This calculator solves for the principal (P) from your desired end balance (A) using the classic relation: A = P(1 + r·t). Enter your end balance, interest rate, and time—get the principal and total interest instantly.
Why this calculator matters
- Accurate planning: Work backward from a goal amount to the starting principal.
- Faster decisions: Instant results without spreadsheets.
- Education-ready: Perfect for students learning simple interest.
- Transparent math: Uses P = A / (1 + r·t); unit conversions handled automatically.
Common use cases
- Figure out the deposit required today to reach a target balance later.
- Estimate the starting balance on simple-interest loans or notes.
- Create quick classroom examples without manual mistakes.
How to use
- Enter the End Balance (A) you want to reach.
- Enter the Interest Rate and select whether it’s per year, month, or day.
- Enter the Term and choose its unit.
- Click Calculate to see the Principal (P) and Total Interest (I).
FAQs
Does this work for compound interest?
No. This tool uses simple interest only. For compounding, use a compound interest calculator.
What’s the difference between principal and end balance?
Principal is your starting amount. End balance is the final amount after interest is added.
Is the rate converted correctly if I choose months or days?
Yes. The calculator converts the term and rate to matching periods before applying the formula.
Which currency does it use?
Results display in your browser’s locale with a USD default symbol. You can treat values as any currency.
Is this accurate for large amounts?
Yes—simple interest scales linearly, so the math is accurate for small or large amounts.